1. Technical Field
The present invention relates to a driving technology for a light emitting device, with which the light emitting element is driven in a pulsed manner.
2. Related Art
Since in image display apparatuses such as laser-scanning displays (see, e.g., JP-A-2007-140009) or laser type projectors, the emission light intensity of the light emitting element is controlled for each of the pixels constituting a picture, pulse width modulation for controlling the duration of supplying the light emitting element with a current in a period (pixel time) for displaying each of the pixels is performed. JP-A-6-013659 is an example of a related art document.
FIG. 1 is a schematic diagram showing an image output unit 10a in an image display apparatus using a laser diode. The image output unit 10a is composed of a grayscale control section 100, a pulse width modulation (PWM) signal generation circuit 200, a light source circuit 300a using a laser diode LD.
The grayscale control section 100 is supplied with image data for designating the display grayscale for each pixel. The grayscale control section 100 is configured as a look-up table (LUT). By reference to the look-up table, the drawing data for designating the emission light intensity of the laser diode for each pixel is generated. The drawing data thus generated is supplied to the PWM signal generation circuit 200. The PWM signal generation circuit 200 generates a PWM signal for controlling the emission light intensity of the laser diode to be the value designated by the drawing data. The PWM signal is a binary signal with an H-level and an L-level respectively designating ON and OFF of the current to be supplied to the laser diode. It should be noted that the grayscale control section 100 and the PWM signal generation circuit 200 are supplied with a drawing clock for defining timing of the pixel time.
The light source circuit 300a has a laser diode LD, a load resistance Ra, and a switching transistor Q1. The laser diode LD, the load resistance Ra, and the switching transistor Q1 are connected in series in this order. The anode of the laser diode LD is connected to a power supply (not shown) for driving the laser diode LD, and the power supply voltage Vcc is applied thereto. The laser diode LD is connected to a supply section of the power supply voltage Vcc and the load resistance Ra via a driving connection line 302. It should be noted that although an n-type MOSFET is used as the switching transistor Q1 in FIG. 1, various types of switching element such as a p-type MOSFET, a bi-polar transistor, or a junction FET can also be used as the switching transistor Q1.
The gate (G) of the switching transistor Q1 is connected to the PWM signal generation circuit 200. When the PWM signal supplied from the PWM signal generation circuit 200 becomes in the H-level to make the gate have the voltage higher than that of the source (S), which is grounded, the switching transistor Q1 becomes in the conduction state (ON state) between the drain (D) and the source (S). Therefore, a current flows through the laser diode LD, the load resistance Ra, and the channel between the drain (D) and the source (S) of the switching transistor Q1, and the laser diode LD emits light. On the other hand, when the PWM signal is switched to be in the L-level, and the gate (G) and the source (S) become in approximately the same level, the channel between the drain (D) and the source (S) of the switching transistor Q1 is switched to be in the non-conductive state (OFF state). Thus, the current stops flowing through the laser diode LD, and the laser diode LD stops emitting the light.
FIGS. 2A and 2B are explanatory diagrams showing a condition of driving the laser diode LD by the light source circuit 300a shown in FIG. 1. FIG. 2A is a graph showing the PWM signal supplied to the light source circuit 300a. FIG. 2B is a graph showing a drive current ILD flowing through the laser diode LD. The horizontal axes of FIGS. 2A and 2B each represent time. The vertical axis of FIG. 2A represents the voltage of the PWM signal, and the vertical axis of FIG. 2B represents the drive current ILD of the laser diode LD.
As shown in FIGS. 2A and 2B, when the PWM signal becomes in the H-level, the switching transistor Q1 becomes in the ON state, and the current flows through the laser diode LD. However, as shown in FIG. 1, the laser diode LD is connected to a supply section of the power supply voltage Vcc and the load resistance Ra via a driving connection line 302. Since the driving connection line 302 has some inductance, even if the voltage applied to the power supply voltage supply section is formed as a rectangular wave, there occurs a delay in increasing the drive current ILD caused by the inductance. As a result, the waveform of the drive current ILD approximates a ramp wave gradually increasing from the leading edge of the pulse toward the trailing edge thereof. Since in general the emission light intensity of the laser diode LD is proportional to the drive current ILD, if the waveform of the drive current ILD becomes the ramp wave, the waveform of the emission light intensity also becomes the ramp wave.
FIGS. 3A through 3C are explanatory diagrams showing an influence exerted on the emission light intensity by the drive characteristic of the laser diode. FIG. 3A shows an ideal laser diode drive characteristic, and FIG. 3B shows the laser diode drive characteristic of the circuit shown in FIG. 1. FIG. 3C is a graph showing a relationship between a pulse width tPW and an average emission light intensity Pa per pixel time when the laser diode is driven with the drive characteristics shown in FIGS. 3A and 3B.
As shown in FIG. 3A, the PWM signal with the rectangular waveform becomes in the H-level during a period tPW corresponding to the average emission light intensity Pa in one pixel time Tp. Further, if the drive of the laser diode is performed ideally, the waveform of the emission light intensity Po of the laser diode becomes a rectangular wave with the same pulse width tPW as that of the PWM signal. On the other hand, as described above, since the waveform of the drive current ILD becomes the ramp wave in the drive circuit shown in FIG. 1, the waveform of the emission light intensity Po also becomes the ramp wave as shown in FIG. 3B.
The total light intensity of the light emitted from the laser diode in one pixel time Tp is obtained by time-integrating the emission light intensity Po in the corresponding pixel time. Therefore, as illustrated with the broken line in FIG. 3C, if the laser diode is driven ideally, the average emission light intensity Pa is proportional to the pulse width tPW. On the other hand, if the drive circuit shown in FIG. 1 is used, since the waveform of the emission light intensity Po becomes a ramp wave, the average emission light intensity Pa varies along the square of the pulse width tPW. As described above, if the relationship between the average emission light intensity Pa and the pulse width tPW is nonlinear, it becomes difficult to control the average emission light intensity Pa in accordance with the grayscale of the image data. Further, the average emission light intensity Pa in the case in which the pulse width tPW becomes the one pixel time Tp, namely the maximum value of the average emission light intensity Pa becomes a half as large as in the case in which the waveform of the emission light intensity Po is an ideal rectangular waveform, if the waveform of the emission light intensity Po becomes a ramp waveform.
As described above, if rising of the drive current ILD becomes slow, rising of the emission light intensity Po becomes slow, it becomes difficult to control (grayscale control) the average emission light intensity Pa in accordance with the display grayscale value, and further, the maximum value of the average emission light intensity Pa is lowered. This problem is a problem common to various light source devices for supplying a light emitting element with a pulse current and controlling the average emission light intensity of the light emitted from the light emitting element.